1. Field of the Invention
The present invention relates to coated fine particles, dispersed fine particles, a method for producing coated fine particles, an ink, a recording method and recorded images, and in particular to coated fine particles containing fine particles as the cores and having a crystal layer of a compound formed on the surface of the fine particles, a dispersion using the coated fine particles, a method for producing the coated fine particles, an ink using the coated fine particles, and a recording method and a recorded image using the ink.
2. Related Background Art
To date, dyes have been used as coloring materials for ink-jet recording liquid (ink) requiring high definition. Ink using a dye provides images with features such as high degree of transparency, high definition and superior color development, but in many cases it is inferior in image fastness such as light fastness and water resistance. In recent years, to cope with the inferiority in light fastness and water resistance of the image, pigment inks have been manufactured that use in place of dyes organic pigments and carbon black as coloring materials. Thus coloring materials used for inks have been shifting from dye to pigment in view of enhancing image fastness, and, for example, the following various proposals have been made.
For example, there have been proposed a compound which is prepared to have a structure with a group compatible with a given solvent and thus is soluble in the solvent, and in which the group compatible with the solvent may be eliminated by retro Diels-Alder reaction resulting in an irreversible decrease in the solubility in the solvent, and an ink using the compound (see Japanese Patent Application Laid-Open No. 2003-327588). When this compound is used as the coloring material, it is dissolved in an ink solvent (i.e., in dye-like state), but it may be made insoluble (i.e., in pigment state) in the solvent to increase image fastness when applied onto a recording material and subjected to retro Diels-Alder reaction. However, this proposed method requires means to apply external energy, such as heating, and light, electromagnetic wave and radiation irradiation, to produce the above reaction of the compound dissolved in a solvent (i.e., in dye-like state) when applied onto a recording material.
In addition, there has been proposed a phase change ink that uses a polymerization compound capable of thermally reversible Diels-Alder reaction as a viscosity temperature control material for an ink-jet ink carrier (see Japanese Patent Application Laid-Open No. H11-349877). This proposed method is disadvantageous in that due to reversible reaction, cooling under a reduced solubility condition can induce cyclization and cause solubility to increase. In addition, there has been proposed a method by which a compound (dye) undergoing retro Diels-Alder reaction is applied onto a recording medium containing a metal compound, and the compound (dye) undergoing retro Diels-Alder reaction is subjected to retro Diels-Alder reaction to form a pigment (see Japanese Patent Application Laid-Open No. 2004-262807). Although the resultant pigment has been converted on the recording medium to a pigment insoluble in the solvent, the resultant image is subject to considerable color irregularities. Examination of the recorded image with various observation apparatus such as X-ray diffractometer revealed heterogeneous pigment formation, mixed crystals and aggregation, and indicated the necessity for single crystallization of pigment to provide satisfactory recorded images.
In addition, there has been proposed a method of controlling polarity (solubility, cohesiveness) that uses decomposition reaction by UV light or heat of a triarylmethane compound, and optically and thermally reversible compounds such as photochromic compounds (see Japanese Patent Application Laid-Open No. H10-31275). Although irreversible state may be formed because the polar region is a system decomposed through radical ion cleavage, oxidation degradation reaction can be induced due to extreme instability of by-product. In addition, because photochromic reaction is a reversible reaction for visible and UV light and heat, maintaining a constant state is difficult.
Furthermore, there has been proposed a method of improving recorded image fastness that causes Diels-Alder reaction of ink when applied onto a recording material (see Japanese Patent Application Laid-Open No. H07-61117). In addition, a method of preventing a yellowing event due to retro Diels-Alder reaction incited by a component of a recording medium has been disclosed that involves potent dienophile contained in the recording medium as a component to produce Diels-Alder reaction (see Japanese Patent Application Laid-Open No. S64-26444).
Some pigments consisting of crystals have two or more crystal types even when the chemical formula, composition and structure are the same, and are referred to as polymorph. Examples include types α, β and ε of phthalocyanine blue, and these have different absorption coefficients and refractive indices and hence different hues and opacifying properties. Organic pigments are not only used in the coating industry as coloring material but also in the electronics industry, for example, as a charge generation agent for electrophotographic photosensitive members, a recording medium coloring matter for CD-R and DVD-R, a coloring agent for toner and ink-jet printer inks, a color filter coloring matter for liquid crystal display devices, and a luminescent material for organic EL devices. To use organic pigments for the uses above, it is first important that they have high purity and specific absorption characteristics. Absorption characteristics depend on the chemical structure, particle size, crystal type and purity of the pigment. Many organic pigments in particular have a plurality of crystal types even when the chemical structure is identical, so ensuring high purity while controlling the crystal type is an important point in developing a novel organic pigment.
For example, various organic pigments have been used as a charge generation material for electrophotographic photosensitive members, and there is a strong need for a pigment having high-sensitivity absorption-characteristics for semiconductor laser light and near infrared light, which represents the emission wavelength of LED light. As an organic pigment meeting this requirement, phthalocyanines have been studied extensively. Phthalocyanines vary in absorption spectrum and photoconductivity according to the crystal type as well as the type of the central metal, and according to some reports a specific crystal type has been selected from phthalocyanines with the same central metal for electrophotographic photosensitive members.
For metal-free phthalocyanines, the crystal type X has been reported to have high photoconductivity, and sensitiveness for near infrared light of 800 nm or more, while for copper phthalocyanines, the crystal type ε among many other crystal types has been reported to be sensitive for the longest wavelength. However, type X metal-free phthalocyanine is a metastable crystal type and difficult to manufacture to achieve stable quality, and stable quality one can be obtained with difficulty. Although the ε type of copper phthalocyanine has high spectral sensitivity for longer wavelengths compared to the α and β types of copper phthalocyanine, the sensitivity drops sharply at 800 nm compared to 780 nm, and this makes it unfit for use with semiconductor laser with fluctuating emission wavelength. It is known that copper phthalocyanine has electrostatic property, dark decay and sensitivity that can vary significantly depending on whether the crystal type is α, β, γ or ε (see, for example, Senryo-to-Yakuhin, Vol. 24, No. 6, p. 122 (1984)), and spectral sensitivity has been also reported to vary because absorption spectrum varies depending on the crystal type (see, for example, Denshi Shashin Gakkai Shi Vol. 22, No. 2, p. 111 (1984)).
Thus the difference in electrical characteristics depending on the crystal type is well known with respect to metal-free phthalocyanines and many other metallophthalocyanines, and much effort has been made to produce a crystal type with satisfactory electrical characteristics. Many pigments are synthesized in water or subsequently treated to form primary particles adjusted for size and shape, and these particles are likely to made coherent in subsequent processes, especially in the drying process, to form secondary particles. It is therefore necessary to deflocculate these particles in the dispersion process.
Examples of general methods for controlling (or refining) the crystal type of an organic pigment include, in addition to the method of controlling the crystal type during the synthesis step, the so-called sulfuric acid method (see Japanese Patent Application Laid-Open No. H05-72773), such as the acid pasting method and the acid slurry method, a method involving dissolving an organic pigment or making it amorphous by grinding methods such as the solvent milling method, the dry milling method and the salt milling method followed by conversion to a desired crystal type (see Shikizai-Kyokai, et al., “41st Ganryo Nyumon Koza-Textbook (1999)”), and a method involving heating dissolution of an organic pigment in solvent under a heating condition followed by slow cooling for crystallization (see Japanese Patent Application Laid-Open No. 2003-160738). In addition, as a method for controlling the crystal type for organic thin film, the method of controlling sublimation temperature to attain a desired crystal type (see Japanese Patent Application Laid-Open No. 2003-003084) is commonly used.
In addition, for metallophthalocyanine compounds, for example, a method has been disclosed in which the central metal is changed to aluminum in the method using copper phthalocyanine as coloring material for the ink used for recording images, in order to improve ozone resistance of recorded images (see, for example, U.S. Pat. No. 4,535,046, Japanese Patent Application Laid-Open Nos. H06-100787, H09-328621 and H11-24132). Furthermore, a method has been disclosed in which a coordinate bond group is linked to the central metal (see, for example, U.S. Pat. Nos. 4,311,775 and 6,153,000, International Publication WO 92/01753). However, in the proposed methods above, it is difficult to increase the purity of the crystal type of resultant metallophthalocyanines and provide crystals of uniform size.